SEISMIC VP & VS TOMOGRAPHY OF TEXAS & OKLAHOMA WITH A FOCUS ON THE GULF COAST MARGIN

It is not clear whether the rifting process that opened the Gulf of Mexico (GoM) and produced Texas’ Gulf Coast Plain (GCP) was “active” or “passive” in nature. A recent study interpreted the GCP to be an “active” volcanic rifted margin using available gravity, magnetic, bore hole and geological data. Older studies favor passive models for the opening of the GoM. A large magnetic anomaly in the GCP suggests that the crust was modified. Previous seismic data were sparse and of limited quality in this region so we deployed a dense transect of broadband seismographs across the GCP to clarify its structure and origin.

Twenty-three broadband seismographs were deployed in a line from Matagorda Island, in the GoM, to Johnson City, TX on the uplifted Llano Plateau from July 2010 to December 2012. Data from this linear array were integrated with data from the EarthScope Transportable Array in Texas, Oklahoma, New Mexico and SIEDCAR (a deployment in west Texas and southeastern New Mexico). Seismic tomographic images were produced using the iterated nonlinear Fast Marching Tomography (FMTOMO) algorithm.

The most striking result of seismic tomography is the marked difference between Vp and Vs perturbations beneath the GCP. Vp is slightly fast, aside from a small shallow anomaly between 50 and 100 km on the densely-instrumented transect and a less pronounced slow anomaly below 360 km depth. These differences become more evident in plots of Vp/Vs, which show elevated ratios beneath the entire GCP that are largely due to unusually low Vs. Other anomalies correlate with surficial tectonic features, including a broad, slow Rio Grande Rift zone and southern Oklahoma Aulacogen, and a fast region that marks the former southern Laurentia. A smaller-scale feature of interest includes a semi-circular ring of slow Vs surrounding a high velocity core that corresponds to the Llano Uplift, part of the Edwards Plateau in central Texas, an outcrop of Grenvillian-age granite.